The invention pertains to a double crucible apparatus for making optical fibers containing fluoride glass, the process for making optical fluoride glasses, and the fluoride glass compositions produced thereby.
In one of its more specific aspects, this invention pertains to fluoride glasses which can be fiberized to produce continuous, defect-free optical fibers. Recently, there have been developed optical fibers which are especially useful in the area of data communication. The data is communicated as light in the near IR region along the optical fiber. A fluoride glass fiber is a particularly attractive candidate for waveguide optical cable systems and various types of transmission systems due to the fluoride glass fibers's ultralow scattering loss property.
Such fluoride glass optical fibers typically comprise an inner, or core, fluoride glass fiber coaxially within an outer, or clad, fluoride glass fiber, It is important that the "core/clad" fluoride glass fibers have certain desirable properties. One of these properties is that the core/clad fiber be capable of being formed in long continuous lengths. Also, it is important that the core/clad fluoride glass fibers have low attenuations or transmission losses in order to allow for the transmission of quality data communications. Further, the transmission of light along the glass fiber is a wave phenomena. Any defects in the glass fiber, such as crystals in the glasses, voids in the core glass or bubbles at the core/clad surface interface, cause scattering of the transmitted light thereby greatly diminishing the quality of the data transmissions.
One drawback in using fluoride glass fibers is that it has previously been difficult to form fluoride glass fibers having an adequate and useful length, while still maintaining the desired low scattering loss property which can be achieved when producing bulk fluoride glass.
A known method used in forming fluoride core/clad glass fibers is the preform process. In the perform process the core part of the fiber is formed by pouring a hot fluoride glass melt into a preformed clad fluoride glass pipe which has been manufactured beforehand. During this process, however, crystals and bubbles are sometimes produced at the core-clad interface, so that the effective length of the glass fiber is restricted. Furthermore, the inner diameter of the clad glass pipe is limited to that size which will allow core glass to be poured into it. The diameter of the clad glass pipe produced by this method is too large to be useful for the forming of single mode fibers.
Another method for forming core/clad fluoride glass fibers is known as the double crucible process wherein one crucible containing a core glass is located within another crucible containing a clad glass. The fibers are thus drawn from the double crucible. However, with respect to fluoride glasses, there are several drawbacks which previously have been associated with the double crucible method. In particular, the viscosity of the fluoride glass at which fibers can be formed lies below the fluoride glass's liquidus temperature. Consequently, crystals can form during the draw process. In order to minimize the growth of these crystals, fibers must be drawn at very high viscosities. However, in order to draw fluoride glass fibers at high viscosities, highly pressurized crucibles must be used in order to maintain the fluoride glass throughtput at a rate which will produce, without fracture and at sufficiently high draw speeds, crystal free optical fibers. However, there is a high viscosity limit at which the fluoride fibers can be drawn wherein the drawn fibers exhibit a circular cross section and are free from surface irregularities. Beyond this viscosity limit, fibers can be drawn but any nonuniformities which occur during the fiberizing process are frozen into the fiber. In addition, at high glass viscosities, the double crucible process previously tended to produce hollow fibers which is a manifestation of the glass's inability to flow sufficiently well to fill the void that is created as glass is removed from the bottom of the crucible during the fiberization process. Another drawback to the previously known double crucible process is that the core/clad fluoride glass fibers thus formed do not have a consistent fiber diameter, which is critical if such fluoride glass fibers are to be used for fiber optics.
The present invention is directed to a solution of these problems.